Ultra-stable atomic clocks find widespread applications in navigation, communications, and scientific measurements and experiments, to name just a few. An atomic clock comprises a gas cell or vacuum tube to confine an ensemble of reference atomic oscillators, isolated from changes in the environment where the clock operates. In many instances, an atomic clock comprises a quartz oscillator, where the frequency of the quartz oscillator is corrected and accurately maintained by exploiting the physics of the reference atomic oscillators. Sampling the output of the quartz oscillator at specific oscillation intervals provides the ticks of the atomic clock.
One type of atomic clock is an atomic ion clock where the two lowest energy levels of the ions determine the frequency of the “atomic oscillators”. The ions are confined within an ion trap by the use of radiofrequency (RF) and static (DC) electric fields. For the two lowest energy levels with an energy difference of ΔE, the frequency ω associated with the ion atomic oscillators is given by ΔE=ω, where  is Planck's constant.
Mercury (Hg) ion atomic clocks offer some advantages over other high-performance clocks being developed today. FIG. 1 illustrates the two lowest energy levels (split from the 2S1/2 energy level) for 199Hg+ ions, labeled 102 and 104, sometimes referred to as the upper clock level and the lower clock level, respectively. A 202Hg discharge lamp may be used to provide ultraviolet light with wavelength 194 nm, so that an ion may be excited from upper clock level 102 to 2P1/2 optically excited state 106. This is pictorially represented by photon absorption line 108, showing that the energy difference between energy levels 106 and 102 corresponds to the energy of a photon with a wavelength of 194 nm. When an ion is in optically excited state 106, it may then transition to lower clock level 104, as pictorially represented by photon emission line 110, emitting a photon at a wavelength smaller than 194 nm. In this way, with ions initially populating energy level 102, fluorescence is observed because the absorption of 194 nm light leads to the emission of light at a lower wavelength (higher frequency).
Once ions are driven into lower clock level 104, they no longer absorb and scatter the 194 nm photons. Fluorescence will resume when an interrogating microwave radiation is tuned to the approximately 40.507 GHz transition between energy levels 102 and 104, thereby leading to a repopulation of ions to upper clock level 102. The fluorescence response peaks when the microwave radiation is at approximately 40.507 GHz, and will decrease as the microwave frequency is tuned away from 40.507 GHz. FIG. 2 illustrates a sample data fluorescence response curve, giving photon count as a function of frequency offset from the center frequency (approximately 40.507 GHz) of the interrogating microwave radiation.
Rather than look directly for a peak fluorescence response, some atomic clocks will modulate the interrogating microwave radiation at two frequencies ν0+Δν and ν0−Δν, and will vary ν0 until the fluorescence response at frequency ν0+Δν is substantially equal to the fluorescence response at frequency ν0−Δν. When this occurs, the two frequencies are essentially centered about the peak fluorescence response frequency, so that ν0 is essentially the peak response frequency and is a measure of the frequency transition between upper and lower clock levels 102 and 104.
The above description of a mercury atomic ion clock may be represented at the system level by FIG. 3, illustrating ion trap 302, with optical windows 304 and 306, and microwave window 308. The source of optical radiation is labeled 310, and optical detector 312 measures the fluorescence. Oscillator 314 provides a reference frequency, which is modulated by modulator 316 to ν0+Δν and ν0−Δν, and microwave radiator 318 interrogates the ions in ion trap 302 via microwave window 308. These component systems are monitored and controlled by control system 320, so that the frequency of oscillator 314 is controlled to provide equal fluorescence responses at the two frequencies ν0+Δν and ν0−Δν. The output of oscillator 314 provides a stable frequency reference to be used as the basis for an atomic clock. (A clock will count the cycles, and add one second after an appropriate number of cycles have been accumulated.)
The atomic clock system of FIG. 3 is operated in two phases, a first phase in which lamp 310 optically stimulates the ions so that optical detector 312 may detect fluorescence, but where microwave radiator 318 is off; and a second phase, where lamp 310 is off and the microwave radiator 318 is on so that upper clock level 102 may be repopulated.
U.S. Pat. No. 5,420,549, hereinafter referred to as the '549 patent, discloses a mercury ion atomic clock in which a linear quadruple ion trap is electrically separated into two regions. The regions are co-linear, have the same number of electrodes, and are driven by the same RF field to contain the ions; but separate DC voltages are applied to each region to shuttle the ions from one region to the other. Fluorescence is stimulated only when the ions are in the first ion trap, whereas the resonance interrogating microwave radiation is applied only when the ions are in the second ion trap region. As explained in the '549 patent, by separating the ion trap into two regions, a resonance region and a fluorescence region, the resonance region may be made much smaller than the fluorescence region, making it easier to magnetically shield the resonance region, as well as simplifying thermal control. Other advantages are described in the '549 patent.